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I'm a newbie in this hobby and last week end I had the opportunity to start testing my setup with guiding.

I read a couple PHD2 tutorials and went on with it, I was successful taking 480sec exposures with round stars, so I pretty happy with my first attempt.

My question is related with the Calibration data produced by PHD2, please check bellow my data:

phd2.png.f6825ff0669f0bb8493e638a151e7568.png

I'm not fully understanding the data..

1) I assume a perfect tuned mount RA and DEC should align with the 90º graph scale.

1.a) Is this the orthogonality error ?

2) If my assumption is right, how do I correct the deviation ?

3) What are the white dots ?

4) Why the read and green lines do not align with the dots ?

5) What should I conclude concerning the performance of my mount by looking at the data ?

 

I know.. a lot of questions..

Just a bit of context, my setup is a NEQ6 with an ED80 (0x85 reducer), for guiding I have a TSL80 (328mm) with an ASI120, btw the NEQ6 is fully stock.

Thanks,

-jb

Edited by 0x0539

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Orthogonality error is how much the two axes are off being at a right angle, although I have never been able to get my head around the way PHD presents this info.  

The angle you mention in (1) is dependant on how your guide camera is inserted.  Twist the camera in the ST80 a bit and see if it improves or gets worse.  Shouldn't make a lot of difference to the guiding, although you will need to recalibrate.

The white dots (3) are the return path (east or south movement).     

(4) Imperfect polar alignment, slight imperfections in the mount/drive train, backlash &c mean the star doesn't always return by the same path.  Try running guiding assistant tovsee what advice it gives, if you can bear to waste ten minutes or more imaging time.

(5) If you're happy with the results, round stars, crisp detail, then no don't be concerned.

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Next time I'll check what the wizard has to tell me but it now makes more sense, if my camera was at a perfect 90º angle them PHD would plot something close to this:

angle.png.8a494aafd45466fdc5082d41405cd90c.png

And it would be possible to see the 6 deg ortogonal error between RA and DEC, the red and green lines are at less than 90 deg apart. Is this physical property of the mount unrelated with the "cone error" ?

I remove the guide camera from the guider scope every time I pack home so it's a good advice to know that it's required to calibrate between sessions if the position of the camera is changed, thumbs up.

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Hi 0x0539 and welcome, just a couple of points that I might throw in for consideration.

Firstly the setup you have using an ST80 guide scope, that's the setup I had initially, but always seemed to have trouble finding guide stars, when I did it was OK at best, but guiding would fail at times. Changed to a 50mm finder guider, this didn't seem sensible, 80mm down to 50mm, but I found guide stars no problem and had it guiding for hours at a time. It may be that the guide camera I have (QHY5-II) suited this size scope better, who knows!

Secondly a program I am considering and have downloaded the software for is SharpCap with polar align, a fellow imager has used this  polar align feature and has had very good results. He has tested this with 9 minute exposures with hardly any star trailing on tracking only using an NEQ6 mount. The initial set up is using the guide camera for polar alignment prior to guiding and its polar accuracy is claimed to be down to seconds of arc.

Check this out on the SharpCap Polar Align web site and have a look at the tutorials, it seems quite simple to use.

PS - I have used PHD2 but as I am more used to PHD1 and I tend to stick to it, it seems less complicated as I am only interested in guiding and not all the bells and whistles that come with PHD2.

Happy imaging !

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I made a mistake, my guider scope is not the Skywatcher but the TS one: TSL80; in the end them both burn down to the same thing as the ST80 is 400mm and the TSL80 is 328mm focal length.

ts-tsl80d-80-mm-leitrohr-sucher-1000.thumb.jpg.b5f3e68332c1b0d176e8e9b87db396f9.jpg

The problem with my setup is that the TSL80+ASI120 has a FOV of 0.84° x 0.63° which is narrower than my imaging setup ED80+D7000 with a FOV of 2.63° x 1.73°; I do believe it should be the other way around for best results.. time will tell how it will perform. For polar alignment, at that night, I was testing Alignmaster and was able to archive a good alignment in the AZ but not so good in ALT which could be related with the orthogonal error I'm seeing in DEC (?).

Could the orthogonal error be less related with Polar alignment but instead more with the angle RA and DEC have when in the park position (weights down, scope up) ?

Edited by 0x0539

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Your guidescope looks fine. The FOV is only due to the small sensor on the ASI120 but as long as you cn find guide stars that is not a problem. 

I'm personally not keen on the adjustable rings and finderscope attachment as the can be a source of flexure but they will do for now. Just make sure everything is nice and tight.

The orthogonality error is likey due to polar alignment error causing drift in declination. The sign of this is that your RA leg does not retrace its West bound steps when returning East

After you've done a first polar alignment, redo your calibration.

When doing your calibration, use the calculator in the Brain, making sure you enter the correct values. The default is for 12 steps. Your calibration took 24 steps which is on the high side. This is not an issue in itself but may indicate some of the input values were incorrect.

As a general rule, you shoudl attach your guide log when you need help with PHD2. It contains a wealth of information for troubleshooting. You can also view it yourself using PHDLogViewer or PEMproLogViewer to analyse your guiding.

You might also consider rotating your guide camera so that it aligns more closely with the RA and Dec axes. This can be useful for troubleshooting later as it makes it easier to tell if things like star trails on your images are in RA or Dec direction.

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This are my settings from the brain dialog box for the calibration steps, I see I had declination degrees at zero, does this make a big change ? All the other parameters seems to be correct for my guiding setup.

brain.png.e0f261249d2381ed202bc1f98f4eff6f.png

Here is the calibration when analyzed by PHDLogViewer:

phdcal.thumb.png.a9b41406df28a1fa0bceca02b6d7864c.png

And then on that night I did three test sessions, the last one using dither.

On session 2/3 I see a large spike in DEC.. what can that be ?

phdsess1.thumb.png.25b803589474f4764238e32e92c75f11.png

phdsess2.thumb.png.f8c0cc12385352979da484b2e4e357e5.png

phdsess3.thumb.png.7c48b631b48e986dd52fe382685cddda.png

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It would be easier and more sueful to attach the guide log instead of loads of screen shots.

One thing I can just amke out by squinting is that your guide rate wuld suggest that it is set to 0.1x rather than 0.2x.

It is odd that PHD2 isn't picking up the correct guide rate and declination in the calculator. The guide rate error explains why you are taking 24 steps instead of 12. The declination error of 0 degrees versus the -22 deg reported in the calibration is not that significant. Unfotunately I can't see what declination is reported whilst guiding on those screen shots.

The spike is most likely some sort of flexure. Hard to say more without more information.

 

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Overall you are getting pretty good performance from your mount. The exception is the big spikes in Dec. In fact they are not spikes but a big movement that PHD2 is able to pull back so it appears as a spike. The movement is only in declination so cable drag is less likely but should not be discounted. I'd start by looking at your balance. Another possibility is the gear mesh. Could be a bit slack giving you a large amount of backlash.

Is a reason why you are guiding at 0.1x sidereal? Most people would use 0.5x or 0.9x.

 

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17 minutes ago, kens said:

Is a reason why you are guiding at 0.1x sidereal? Most people would use 0.5x or 0.9x.

I can't recall where did I get this info from but I read somewhere that it was advised to use 0.20x sideral as guiding speed. Should I update the value on the calculator to be 0.5x ?

Last weekend I've modded my EQ6 to the Rowan belt kit, I'm hoping the next weekend, or the one after it, to test it's performance.

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You would need to change it in EQMOD. I thought PHD2 would feed the value from EQMOD into the calculator but I could be mistaken. Whatever value you choose in EQMOD should also be entered into the calculator.

I'm also in the middle of belt-modding my EQ6. It should reduce the signature EQ6 122 second periodic error. This is quite prominent on your RA  graph. 

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Oh ! I see in EQMOD ASCOM PulseGuide Settings which are set to 0.1 for both RA and DEC, will set them to 0.5x.

Should I also update the min pulse width from the default 20ms and the DEC backlash from 0ms ?

I've also tried PEM PRO and I can see the biggest spike is ~121 which is identified as Stepper Gear, just for the sake of curiosity since I'm now using belts.. would it be possible to physical tune this frequency on the mount by for instance adjusting the position of the stepper in relation to the idler gear ?

Edited by 0x0539

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I am closely watching this thread as I am trying to iron out my neq6 graph.

You mentioned that you remove the guide cam from the guidescope. I would not do this if possible and leave as much intake as you can. If you need to remove image or guide cam then I would use a tipex pen and mark both the unit and mounting so you have two points of ref when it goes back together.

Have you tried using sharpcap's polar align as you might find this a quick and accurate way to polar align.

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15 minutes ago, spillage said:

sharpcap's polar align

I have been using a trial version of AlignMaster.

I was unaware of it, just saw a youtube video about the feature and it seems very simple to use.

I don't have a CCD on my imaging scope, I have a DSLR and Sharpcap will not connect to it would this mean I would have to *really* align my guiding scope with the imaging scope to use this feature ?

 

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sharpcap will use your guide cam and scope to align not you main scope and camera.

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1 hour ago, 0x0539 said:

Oh ! I see in EQMOD ASCOM PulseGuide Settings which are set to 0.1 for both RA and DEC, will set them to 0.5x.

Should I also update the min pulse width from the default 20ms and the DEC backlash from 0ms ?

I've also tried PEM PRO and I can see the biggest spike is ~121 which is identified as Stepper Gear, just for the sake of curiosity since I'm now using belts.. would it be possible to physical tune this frequency on the mount by for instance adjusting the position of the stepper in relation to the idler gear ?

Leave the min pulse width and DEC backlash at their default values in EQMOD. Use the Guiding Assistant tool in PHD2 to work out out your PHD2 settings.

With the belt mod the 121 second spikes should be much reduced - at least thats what Im hoping for with mine. You should also have less Dec backlash.

With the normal gears, adjusting the mesh between the idler gear and stepper should help although I've never got it right. Apparently the trick is to use a piece of aluminium foil to get the spacing right. However, its almost impossible to adjust the mesh between the idler and the spur gear on the worm shaft as it is hidden away out of sight save for a tiny peephole in the casing.

 

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